In a time division duplex (TDD) communication system based on OFDM, frequency domain scheduling may be used to improve the communication performance of the system. Frequency domain scheduling refers to a method for fully using the frequency selectivity in a wideband communication system by selecting appropriate sub-band for the transmission of data of user terminals.
With reference to FIG. 1, there is shown a schematic view of the relative power of the sub-carriers received by different user terminals. Each sub-carrier received by a different user terminal will get different relative power when compared to the same reference value. The user 1 has higher relative power on the 30th to 120th sub-carriers, while the user 2 has higher relative power on the 120th to 160th sub-carriers as well as on the 240th to 280th sub-carriers. When resources are being allocated, the 30th to 120th sub-carriers will be allocated to user 1, and the 120th to 160th and the 240th to 280th sub-carriers will be allocated to user 2. Thus, each of the users always selects the sub-bands most suitable for transmission to transmit data, obtaining multi-user diversity gains. When there are a sufficient number of user terminals, a user terminal can always be found for any one of the sub-bands, and the sub-band will be allocated to the user to maximize the power of the sub-carrier, hence exploiting the communication capabilities of the wireless channel to the greatest extent.
In the TDD system, the linear spatial domain pre-coding/beam-forming technology may be used to improve the performance of the system. The linear spatial domain pre-coding refers to the technology that when there are multiple transmitting antennas at the transmitting end, the data flow is mapped through a linear pre-coding operation onto the multiple antennas so as to be transmitted.
With reference to FIG. 2, there is shown a schematic view of the linear spatial domain pre-coding operation. A total number of L data streams X are transformed through a pre-coder into corresponding signals Y to be transmitted on a total number of M antennas. The linear spatial domain pre-coding operation can be seen as a matrix operation, Y=VX, where V is the preceding matrix, the dimension of X is L×1, the dimension of Y is M×1, and the dimension of V is M×L. When L, i.e. the total number of the data stream is 1, the linear spatial domain pre-coding operation is beam-forming. The pre-coding matrix V is calculated from the channel impulse response matrix.
When a base station employs a manner which bases on the channel sounding to obtain a pre-coding matrix, the base station calculates the channel impulse response matrix according to the non pre-coded uplink reference symbol transmitted by the user terminal, and then calculates the linear pre-coding matrix according to the channel impulse response matrix. When such a manner is used, both the downlink reference symbol and the data symbol, which are transmitted to the user terminal by the base station, perform the linear spatial domain pre-coding process. The user terminal does not need to know the pre-coding matrix and does not need to estimate the channel response between each of the transmitting and receiving antennas, the user terminal only needs to estimate the equivalent channel response matrix, which is combined from the pre-coding matrix and the channel matrix, in order to demodulate the data symbols, thus effectively reducing the cost of the reference symbol of the transmitting terminal while supporting any type of pre-coding operation (including beam-forming). At the same time, such an implementation method can quickly track the channel response property in real-time while not approximating the calculated pre-coding matrix, hence having no pre-coding performance loss resulted from approximation. However, the user terminal using such a method can not calculate the indication of the channel quality according to the reference symbol after the pre-coding, resulting in loss of performance in the frequency domain scheduling of the system.
A base station of the prior art may also employ feedback-based manner to obtain a pre-coding matrix, wherein the information on the channel state and the pre-coding matrix are fed back by a user terminal. In such a method, the user terminal needs to feed back a large quantity of information, and the fed back information may be damaged and sometimes may be mistakenly transmitted, resulting in lower performance of the pre-coding/beam-forming of the system.